Fuel cells for combining hydrogen and oxygen to produce electricity are well known. A known class of fuel cells includes a solid oxide electrolyte layer through which oxygen anions migrate; such fuel cells are referred to in the art as “solid-oxide” fuel cells (SOFCs). Another known class of fuel cells includes a membrane through which protons migrate; such fuel cells are referred to in the art as “proton exchange membrane” fuel cells (PEMFCs). Other known classes of fuel cells may comprise phosphoric acid, solid polymer, molten carbonate, alkaline, direct methanol, regenerative, zinc air, and protonic ceramic. The present invention should be understood to comprehend all classes of fuel cells including, for example solid oxide fuel cell, proton exchange membrane, phosphoric acid, solid polymer, molten carbonate, alkaline, direct methanol, regenerative, zinc air and protonic ceramic fuel cells. However, for simplicity, the discussion below is specific to SOFCs.
In some applications, for example, as an auxiliary power unit (APU) for an automotive vehicle, an SOFC stack assembly is preferably fueled by “reformate” gas, which is the effluent from a catalytic gasoline oxidizing reformer. Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen.
The reforming operation and the fuel cell operation may be considered as first and second oxidative steps of the liquid hydrocarbon, resulting ultimately in water and carbon dioxide. Both reactions are exothermic, and both are preferably carried out at relatively high temperatures, for example, in the range of 650° C. to 900° C.
A fuel cell system may be considered to be a chemical engine comprising a plurality of components, sub-assemblies, and sub-systems joined together mechanically and electrically to provide the desired flow paths and control pathways for the fuel, combustion air, reformate, spent gases, cooling gases, and electric current. Typically, there are at least four basic sub-systems: Air Handling, which includes reformer air, cathode air, and cooling air; Reformer, which includes fuel handling and thermal management; Power Electronics, which includes output power conditioning; and Customer Interface. In individual applications, additional sub-systems may be required and/or functional assignments may be grouped differently.
Electronic control is required to manage the various sub-systems to effect the energy conversion process. In the prior art, the sub-systems typically are managed by a distributed control system wherein each sub-system is controlled by a separate micro-controller programmed with algorithms specific to that sub-system. The various micro-controllers are linked by a common communication link for sharing data. This architecture can be useful during development phases of the various sub-systems of an SOFC, facilitating independent development of each sub-system. However, in a fully-developed fuel cell system, this architecture has several drawbacks.
First, parasitic energy losses can be relatively large, as each micro-controller requires a separate regulated power supply between the voltage generated by the fuel cell system and the voltage required for the micro-controller.
Second, a distributed control system may require separate diagnostic and development tools for each sub-system.
Third, a distributed control system may require separate thermo and electrical interfaces for each micro-controller.
Fourth, a distributed control system requires separate paths or bus links between micro-controllers. A typical serial data link can introduce latency delays of up to 100 msec, depending upon the volume of bus traffic.
Fifth, a distributed control system typically requires separate mechanical packages and separate DC power regulator integrated circuits for each micro-controller, and the various controllers must adapt to various ambient thermal and electrical conditions. The various packages may be in different physical locations, making joint servicing or maintenance difficult.
What is needed in the fuel cell system art is a means for integrating the control functions.